1. Field of the Invention
The present invention relates to a drive apparatus, and more particularly, to a drive apparatus suitable to realize a small-sized stepping motor with increased output.
2. Description of the Related Art
Conventionally, stepping motors have been widely used as driving sources for various apparatuses. As a first conventional example, there has been proposed a stepping motor which has a small diameter centering on a rotary shaft of the motor and has high output (see Japanese Laid-Open Patent Publication (Kokai) No. H09-331666, for example).
FIG. 13 is an exploded perspective view showing the construction of the stepping motor according to the first conventional example, and FIG. 14 is a longitudinal sectional view showing the internal construction of the stepping motor in an assembled state.
As shown in FIGS. 13 and 14, the stepping motor is comprised of a first stator 204, a second stator 205, a coupling ring 207, an output shaft 206, and a rotor 201.
The first stator 204 and the second stator 205 are each made of a soft magnetic material and opposed to each other with a predetermined gap therebetween as viewed in the axial direction of the motor (i.e. the direction in which the output shaft 206 of the motor extends). The coupling ring 207 is made of a non-magnetic material and holds the first stator 204 and the second stator 205 with the predetermined gap. The output shaft 206 is rotatably supported by a bearing 204E of the first stator 204 and a bearing 205E of the second stator 205. The rotor 201 is made of a magnet (permanent magnet) and fixed to the output shaft 206. This magnet is circumferentially divided into four pieces and alternately magnetized to different polarities. That is, the magnet is magnetized to four poles.
The first stator 204 having a tooth-shaped end is comprised of first outer magnetic pole portions 204A and 204B which are opposed to an outer peripheral surface of the rotor 201 with a predetermined gap therebetween, and first inner magnetic pole portions 204C and 204D which are opposed to an inner peripheral surface of the rotor 201 with a predetermined gap therebetween. The second stator 205 having a tooth-shaped end is comprised of second outer magnetic pole portions 205A and 205B which are opposed to the outer peripheral surface of the rotor 201 with a predetermined gap therebetween, and second inner magnetic pole portions 205C and 205D which are opposed to the inner peripheral surface of the rotor 201 with a predetermined gap therebetween.
On the first inner magnetic pole portions 204C and 204D, a first coil 202 for exciting the first stator 204 is wound adjacent to one side of the rotor 201 as viewed in the axial direction of the motor. Also, on the second inner magnetic pole portions 205C and 205D, a second coil 203 for exciting the second stator 205 is wound adjacent to the other side of the rotor 201 as viewed axially of the motor.
In the stepping motor constructed as described above, to rotate the rotor 201, the energizing direction of the first coil 202 and that of the second coil 203 are sequentially switched. As a result, the polarities of the first outer magnetic pole portions 204A and 204B, the first inner magnetic pole portions 204C and 204D, the second outer magnetic pole portions 205A and 205B, and the second inner magnetic pole portions 205C and 205D are sequentially switched, causing the rotor 201 to rotate.
In the stepping motor, a magnetic flux generated by energization of the coil flows from the outer magnetic pole portion to the inner magnetic pole portion opposed to the outer magnetic pole portion, or from the inner magnetic pole portion to the outer magnetic pole portion opposed thereto, thereby acting efficiently on the magnet located between the outer magnetic pole portion and the inner magnetic pole portion. Further, the distance between the outer magnetic pole portions and the inner magnetic pole portions can be reduced to the order of the thickness of the magnet having a cylindrical shape, making it possible to reduce the resistance of a magnetic circuit formed by the outer magnetic pole portion and the inner magnetic pole portion. Accordingly, many magnetic fluxes can be generated with a small amount of electric current, resulting in an increased output.
As a second conventional example, a stepping motor with a reduced outer diameter has been proposed (see Japanese Laid-Open Patent Publication (Kokai) No. 2002-142431, for example).
This stepping motor is comprised of a long yoke, a short yoke, a bobbin case, a first stator, a rotor, and a case.
The first stator is constructed to have the bobbin inserted in the long yoke, the bobbin case, and the short yoke. The long yoke has an annular portion and sawtooth-shaped pole teeth protruding from an outer peripheral edge of the annular portion. The pole teeth of the short yoke are shorter than those of the long yoke. The long yoke and the short yoke are constructed such that their pole teeth are pointed in the same direction, each pole tooth of the short yoke is located between the pole teeth of the long yoke, and their ends coincide in position with one another. A cylindrical portion through which a pair of ring-plate shaped flanges are connected is provided between the long yoke and the short yoke. The cylindrical portion supports an annular bobbin case in which a coil held in a wound state is sandwiched.
The second stator is identical in construction with the first stator. The first and second stators are disposed to face each other, with their respective short yokes located thereinside and with their pole teeth facing each other. The rotor is disposed in the central hollows of the first and second stators. The case covers outer peripheral surfaces of the first and second stators.
According to the stepping motors disclosed in Japanese Laid-Open Patent Publication (Kokai) Nos. H09-331666 and 2002-142431, the axial lengths of the motors are determined depending on the heights of the two coils, the length of the magnet, and the thicknesses of the stators. Hence, the stepping motors are long in axial length. On the other hand, if the axial lengths of the stepping motors are shortened, output considerably decreases.
In the stepping motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-142431, a magnetic flux from the long yoke flows toward the short yoke. Since the shorter the distance, the more easily a magnetic flux can flow, most of magnetic fluxes pass through the vicinity of root portions of the short and long yokes which do not face the outer peripheral surface of the magnet, resulting in unsatisfactory magnetic efficiency and low output.
The assignee of the present application has proposed a stepping motor which solves the above described problems (see Japanese Laid-Open Patent Publication (Kokai) No. 2003-023763, for example).
This stepping motor is comprised of a rotor, a first outer magnetic pole portion, a first inner magnetic pole portion, a second outer magnetic pole portion, and a second inner magnetic pole portion.
The rotor is disposed for rotation and has a cylindrical magnet which is circumferentially divided and alternately magnetized to different polarities. The first outer magnetic pole portion, which is excited by a first coil, faces the outer peripheral surface of the magnet within a predetermined first angular range. The first inner magnetic pole portion, which is excited by the first coil, faces the inner peripheral surface of the magnet. The second outer magnetic pole portion excited by a second coil faces the outer peripheral surface of the magnet within a predetermined second angular range. The second inner magnetic pole portion excited by the second coil faces the inner peripheral surface of the magnet. The first outer magnetic pole portion and the second outer magnetic pole portion are arranged on the same circumference centering on the magnet.
The stepping motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2003-023763 can be shortened in axial length. This stepping motor, however, is constructed such that all the outer magnetic pole portions are arranged inside the coils as viewed in the radial direction of the stepping motor. For this reason, when an attempt is made to reduce the outer diameter of the stepping motor, a range in which the outer magnetic pole portions are opposed to the outer peripheral surface of the magnet is limited (i.e., there is a wide area of the outer periphery of the magnet to which the outer magnetic pole portions are not opposed), resulting in reduced output.
The stepping motors disclosed in Japanese Laid-Open Patent Publication (Kokai) Nos. H09-331666 and 2003-023763 are each required to have a predetermined gap between the inner peripheral surface of the magnet and the inner magnetic pole portions opposed thereto, and the gap management at the manufacturing process results in an increase in costs. Further, the stator needs to be provided with the cylindrical inner magnetic pole portions and outer magnetic pole portions. On the other hand, it is difficult to integrally construct the inner magnetic pole portions and the outer magnetic pole portions in the part manufacturing process. If these magnetic pole portions are manufactured separately, and thereafter integrally assembled, the number of parts is increased and cost is increased.
On the other hand, a camera blade drive mechanism employing a stepping motor which is high in magnetic efficiency and short in axial length has been proposed (see Japanese Laid-Open Patent Publication (Kokai) No. H10-62836, for example).
The camera blade drive mechanism is comprised of a bottom plate, a light-shielding blade, a rotor, a drive member, two yoke members, a coil, and a magnetic material member.
The bottom plate is a member which has an exposure aperture. The light-shielding blade is comprised of at least one blade which opens and closes the exposure aperture. The rotor has a permanent magnet which is multipolar-magnetized and disposed laterally of the exposure aperture with a rotary shaft thereof disposed parallel with an optical axis. The drive member is operated with rotation of the rotary shaft and causes the light-shielding blade to open and close. The two yoke members are so arranged as to sandwich the rotator, and one end of each yoke is opposed to the peripheral surface of the rotor over a predetermined angle range. The coil is wound around at least one of the two yoke members. The magnetic material member magnetically couples the other ends of the respective two yoke members to each other and has at least one overhang portion.
In a state where the stepping motor stops rotating, the overhang portion of the magnetic member extends to the vicinity of a magnetic pole of the rotor different from a magnetic pole to which one end of each yoke member is opposed. Specifically, when the stepping motor stops rotating, the magnetic pole of the rotor which is not opposed to one end of each yoke member is positively used so as to improve magnetic efficiency.
Although the stepping motor disclosed in Japanese Laid-Open Patent Publication (Kokai) No. H10-62836 is short in axial length, it is constructed to include a connecting yoke that connects the two yoke members together, and hence the number of parts is increased and cost is high. In addition, magnetic efficiency is poor since a magnetic loss occurs due to connection of the yokes.
Also, a magnetic flux from one end of one of the two yoke members flows toward the overhang portion of the magnetic member and one end of the other yoke, and hence the magnetic circuit is long in length and high in magnetic resistance. For this reason, magnetic efficiency is poor.
Further, since only one of the two coils is energized in running the stepping motor, output is remarkably lower as compared with an ordinary two-phase stepping motor in which alternating current is passed through two coils at the same time.